Nikon D90 has five flash sync modes but not all of the sync modes are available in every exposure modes. Depending on which exposure mode you are currently using, you can choose one of the available flash sync mode. These flash sync modes can be set by using a quick setting screen on the back of the camera or holding down the flash button on the front of the camera (near the lens mount on the left side) and rotating the main command dial (also known as the rear command dial) while the button is still being pressed. When you select a sync mode, you can see the currently selected mode on the LCD screen indicated by the corresponding icon as shown below.

We have already discussed about the front curtain sync and the rear curtain sync in detail in my previous blog post. And, in this blog post, I will describe all of the flash sync modes in brief and will see which mode is available in which exposure setting.

1. Front Curtain sync

In this mode, flash fires as soon as the front curtain opens completely. The shutter then remains open for the duration of the exposure, which is until the closure of the rear curtain. If you are shooting a moving subject, it will produce a stream of motion blur lights effects, also known as the ghost effect, in front of the subject. The reason behind this effect is - when the flash is fired at the beginning of the exposure, it freezes the subject with enough lights in the scene and then subject continues to move, which creates a blurred effect.

2. Rear Curtain sync

When the flash is in rear sync mode, the front curtain opens completely and then remains open for the duration of the exposure. At the end of the exposure, the flash is fired and the rear curtain closes. If the subject is moving, you will get the ghost effect behind the subject because the flash light at the end of the exposure will freeze the subject.

3. Red-eye reduction

In this mode, there is a one second lag after pressing the shutter release button and before the picture is actually taken. During the delay, camera’s red-eye reduction lamp lights which causes the subject's pupil to contract and thus reducing the potential red-eye effects.

4. Slow sync

Slow sync mode allows the camera to use the shutter speed as slow as 30 seconds when the flash is attached, which helps to balance the exposure of the overall scene by illuminating the background with the ambient light and the subject with flash light. It's better to use a tripod to avoid the camera shake if you are using a flash in slow sync mode.

5. Red-eye reduction with slow sync

This mode combines the slow sync with the red-eye reduction behavior when using the Program or the Aperture priority exposure mode.

Which sync mode can be used with which exposure mode?

In Program, or Aperture Priority mode, you can use all five flash sync modes. In Shutter Priority, or Manual Exposure mode, you can use front curtain sync, rear curtain sync and red-eye reduction modes. And in auto, or portrait, or closeup, or scene mode, you can only use auto (same as front curtain sync) and red-eye reduction mode, whereas in Night Portrait mode, you can only use slow sync and red-eye reduction with slow sync mode.